This invention pertains to molybdenum-titanium hermetic members having a thermal expansion coefficient compatible with alumina and other ceramics, and more particularly is concerned with sintering aids for such members.
Electrical discharge devices, such as high pressure sodium vapor arc lamps, commonly utilize transparent or translucent high temperature refractory tubes composed of alumina, yttria, or other ceramics. In the case of an arc lamp, an electric arc extends between two tungsten electrodes within the tube. Current is conducted to the electrodes through the ends of the tube by hermetically sealed feedthrough assemblies. Niobium (Nb) members have been used for this purpose as alumina and niobium have nearly equal thermal coefficients of expansion. The joint between the Nb metal and the alumina is typically filled with a meltable frit based on calcium aluminate and fired. The feedthrough assembly not only conducts electrical current through the end of the tube, but also seals the discharge tube.
While Nb is physically satisfactory as a closure member for ceramic tubes, it is a relatively expensive metal and is in potentially short supply under certain world conditions.
U.S. Pat. No. 4,366,410 divulges an alternate approach wherein the niobium is replaced with a solid solution of molybdenum and titanium. This alloy has a thermal expansion coefficient matched to the ceramic tube. U.S. Pat. No. 4,334,628 teaches the use of metallic sintering aids selected from the group consisting of nickel, cobalt, and copper (Ni, Co, Cu), and mixtures thereof. These metallic elements react with titanium to form intermetallic compounds (e.g., Ti.sub.2 Ni in the case of Ni) which melt at a lower temperature than the sintering temperature of the Mo-Ti alloy alone and, therefore, promotes sintering to hermeticity. Unfortunately, the phases of the intermetallics may have thermal expansion coefficients that are considerably higher than that of single-phase solid solutions of Mo-Ti. When the lamp is energized, the feedthrough temperature rises from room temperature to 800.degree. C. A large difference between the thermal expansion coefficients can lead to stresses and cracking, eventually causing the feedthrough member to lose its vacuum-tight property.